In general, a rise of the tire temperature of a pneumatic tire is considered to be unfavorable from the viewpoint of durability because such rise may accelerate change over time such as deterioration of the material properties of the tire, or may cause breakage of its tread portion at the time of high speed traveling. Especially, for off-the-road radial tire (ORR) and truck/bus radial tire (TBR) for use under heavy load, and run-flat tire at the time of driving with a puncture (with 0 kPa internal pressure), reduction of the tire temperature in order to improve the durability of the tire has been a great challenge.
For example, a pneumatic tire with the following configuration has been disclosed: the tire thickness is increased outward in the tread width direction in a neighborhood of a position where a bead portion is in contact with a rim flange, and the thickened reinforced portion is formed to have such a shape as to cover the rim flange (so-called rim guard) (Japanese Patent Laid-Open No. 2006-76431). According to this configuration, the tire temperature can be reduced by suppressing deformation of the tire surface (especially bead portion) of the sidewall portion.
The conventional pneumatic tire described above, however, has the thick bead portion and the temperature thereof will be increased due to its thickness. Thus, deformation of the bead portion due to load to the tire may break the reinforced portion, and the neighborhood of the bead portion may be damaged by development of cracking caused by this breakage.
Especially, the heavy-duty tire has significant deformation when a heavy load is applied to the tire, thus, providing such reinforced portion creates concerns about the above problem. With this heavy-duty tire, even if the bead portion is not provided with the reinforced portion, the bead portion is originally formed with a greater thickness than that of the tire surface of other sidewall portions, thus the temperature of the bead portion is increased, and not only the durability of the bead portion but also the durability of the tire is reduced.
The present invention has been made in view of such background, and it is an object of the invention to provide a pneumatic tire capable of reducing the tire temperature, particularly in the neighborhood of the bead portion, thereby increasing the durability of the tire.